Refactor repository layout and convert build system to Mage.

This commit implements a massive refactor of the repository, and
moves the build system over to use Mage (magefile.org) which should
allow seamless building across multiple platforms.

tools/vendor/github.com/alecthomas/gometalinter/directives.go

@@ -5,6 +5,7 @@ import (
 	"go/ast"
 	"go/parser"
 	"go/token"
+	"os"
 	"sort"
 	"strings"
 	"sync"
@@ -67,11 +68,12 @@ func newDirectiveParser() *directiveParser {
 // IsIgnored returns true if the given linter issue is ignored by a linter directive.
 func (d *directiveParser) IsIgnored(issue *Issue) bool {
 	d.lock.Lock()
-	ranges, ok := d.files[issue.Path]
+	path := issue.Path.Relative()
+	ranges, ok := d.files[path]
 	if !ok {
-		ranges = d.parseFile(issue.Path)
+		ranges = d.parseFile(path)
 		sort.Sort(ranges)
-		d.files[issue.Path] = ranges
+		d.files[path] = ranges
 	}
 	d.lock.Unlock()
 	for _, r := range ranges {
@@ -204,10 +206,16 @@ func filterIssuesViaDirectives(directives *directiveParser, issues chan *Issue)
 
 func warnOnUnusedDirective(directives *directiveParser) []*Issue {
 	out := []*Issue{}
+
+	cwd, err := os.Getwd()
+	if err != nil {
+		warning("failed to get working directory %s", err)
+	}
+
 	for path, ranges := range directives.Unmatched() {
 		for _, ignore := range ranges {
 			issue, _ := NewIssue("nolint", config.formatTemplate)
-			issue.Path = path
+			issue.Path = newIssuePath(cwd, path)
 			issue.Line = ignore.start
 			issue.Col = ignore.col
 			issue.Message = "nolint directive did not match any issue"